Prolactin is the hormone nobody thinks about until it ruins their life. A 199-amino-acid peptide hormone secreted primarily by lactotroph cells in the anterior pituitary gland, it is best known for its role in lactation -- the very name comes from the Latin pro lactatio, "for milk." But reducing prolactin to a breastfeeding hormone is like reducing testosterone to a muscle hormone. Prolactin has over 300 documented biological functions across vertebrates, touching reproduction, immune regulation, metabolism, osmoregulation, behavior, and brain development. It is the Swiss Army knife of the endocrine system, quietly participating in more physiological processes than any other pituitary hormone.
What brings prolactin into the world of psychoactive substance culture is not what it does at normal levels, but what happens when those levels go wrong. Hyperprolactinemia -- chronically elevated prolactin -- is one of the most common endocrine side effects of psychoactive drug use. Antipsychotics, opioids, SSRIs, and certain stimulants all raise prolactin through their effects on dopamine signaling, because dopamine is the master brake on prolactin release. When that brake fails, the consequences are tangible and distressing: sexual dysfunction, gynecomastia, galactorrhea, menstrual disruption, infertility, bone loss, and a pervasive fatigue that patients describe as feeling "biochemically wrong." In the bodybuilding and performance-enhancing drug community, prolactin management is an entire subfield -- 19-nor anabolic steroids like trenbolone and nandrolone are notorious for raising prolactin, and cabergoline has become as standard in steroid cycles as the steroids themselves.
The dopamine-prolactin axis is one of the cleanest examples of tonic inhibition in neuroendocrinology. Unlike most hormones, which require a releasing factor to enter the blood, prolactin is constitutively secreted -- the pituitary wants to release it all the time, and only the constant drip of dopamine from the tuberoinfundibular pathway keeps it in check. Block that dopamine signal, and prolactin floods the system. This is why virtually every dopamine-blocking drug on Earth raises prolactin, and why dopamine agonists like cabergoline and bromocriptine can bring levels crashing back down. Understanding prolactin is understanding dopamine from the other side of the equation.